Aquarium filter



FIG. 1 is a perspective view of an aquarium filter, showing my new design;

FIG. 2 is a top plan view;

FIG. 3 is a front elevational view;

FIG. 4 is a right side elevational view;

FIG. 5 is a rear elevational view; and

FIG. 6 is a bottom plan view. 

The ornamental design for an aquarium filter, substantially as shown. 